1. Field of the Invention
The present invention relates to an electric power steering control apparatus which outputs an assist force from a motor to a steering system of a vehicle in accordance with the steering torque applied by a driver of the vehicle to a steering wheel, and more particularly, it relates to a new technique for limiting a voltage impressed to the motor in an appropriate manner upon occurrence of abnormality in a motor current control system.
2. Description of the Related Art
There has hitherto been known an electric power steering control apparatus which, in order to generate an optimal steering assist force from a motor, is provided with various kinds of sensor sections (a torque sensor, a motor current detection circuit, etc.), a target current calculation section, a maximum current calculation section, and a current control section, and the target current calculation section calculates various kinds of target currents for the motor based on a steering torque signal, a vehicle speed signal, a motor voltage signal, and so on (see, for example, a first patent document: Japanese examined patent application publication No. 11-147479).
In addition, to make the motor generate an appropriate steering assist force even if abnormality occurs in the motor voltage signal or the like, the maximum current calculation section serves to limit the individual target currents in such a manner that the sum total of the individual target currents becomes an upper limit value of predetermined target currents based on the steering torque signal, etc.
As a result, in cases where the torque sensor or a motor terminal voltage detection circuit fails, the target current of the motor is limited so as to maintain an appropriate steering assist force from the motor.
Further, in order to make the motor current detected by the motor current detection circuit coincide with the target motor current, the current control section determines a voltage to be impressed to the motor while performing feedback control thereof.
In the above-mentioned known electric power steering control apparatus, there is no concept of limiting the polarity of the impression voltage, and hence, no provision is made for limiting the impression voltage. As a result, there is the following problem. That is, when there occurs a failure of the motor current detection circuit, miscalculation of control values or quantities for motor current feedback control, etc., the current supplied to the motor is interrupted. However, there is a possibility that an inappropriate voltage continues to be impressed to the motor over a period of time until the current supply to the motor is actually interrupted or stopped.